Surface treatment to passivate sublimation surfaces to hydrophobic contaminants

ABSTRACT

The sublimation capability of a porous metal plate used in a sublimator is maintained by heating the porous metal plate at an elevated temperature in a flowing stream of oxygen for an extended period of time to form an oxidized surface. A short chain molecule, such as citric acid, is attached to the oxidized surface having multiple functional carboxylate groups and no hydrophobic tail.

BACKGROUND

A sublimator is a primary cooling device for the Extravehicular MobilityUnit (EMU) Space Suit and for the Orion Spacecraft. Its functional heartis a metallic porous plate that is exposed to space vacuum on one side.It is supplied with expendable feed-water on the other side. Inoperation, the feed-water freezes on the porous plate surface, and thevacuum side progressively sublimes water from this ice to the vacuum ofspace as waste heat is introduced into the plate.

A sublimator has two fluid loops inside. Both loops are closed. One looptakes fluid from a heat source and the other takes fluid to the porousplate surface. There is no mixing of fluids and the sublimation looptakes heat by metal to metal contact.

Experience has shown that trace contaminants in feed-water (primarilyamphipathic long-chain organic acids, fatty acids and surfactants withcarboxylate functional groups form a sublimation impeding molecularmonolayer that can drastically impact the sublimation process andtherefore sublimator performance. Trace processing aids, as exemplifiedby abietic acid from the early EMU Neoprene Latex feed-water bladders,sodium dodecyl benzene sulfonate (a candidate soap for InternationalSpace Station-ISS processed water), and acrylic acid oligomers from oneof the current operational ISS Water Processor multi-filtration bedsorbents have exhibited this phenomena.

It is believed that the hydrophilic “heads” of the amphipathic moleculesanchor to available cationic charge on the metallic surface of theporous plate, and that the hydrophobic “tails” of the amphipathicmolecules form an intertwined monolayer much the same as a lipid bilayerin a cell. This phenomenon does not occur with non-volatile contaminantsthat do not have the structural features of an amphipathic molecule suchas corrosion products, iodine and biofilm. These contaminants merelydislodge from the effluent side of the porous plate during thesublimation process and have minimal effect on performance.

The sensitivity of a sublimator to trace amphipathic molecules that formmonolayers (very common processing aids for non-metallic materials)severely restricts the types of materials that can be used to containand/or transfer feed-water, and it restricts the sources of thefeed-water. This has becomes a serious logistics challenge on long-termmissions such as on the International Space Station where recycled waterthat contains trace amounts of acrylic acid oligomer from one of theWater Processor Assembly sorbents is the preferred source of EMUfeedwater. What is needed in the art is a means to passivate asublimator surface to the anchoring of amphipathic contaminants.

SUMMARY

This invention involves a two-step process to passivate a porous platemetallic surface and prevent metal cation charges from forming, so as toprevent anionic carboxylate “heads” of amphipathtic contaminates fromanchoring on the plate. Since the hydrophobic tails are not fixed inplace, they do not intertwine to form a structured sublimation impedingmonolayer.

The first step comprises a high temperature oxidation of the metallicplate surface in a flowing stream of oxygen for a minimum of four hours.This step converts most of the positively charged cations of the metalcationic surface to an uncharged oxide.

The second step comprises adding a short-chain molecule with multiplefunctional carboxylate groups and no hydrophobic tail to render thebalance of the metal cation groups passive, or unavailable forcarboxylate anchoring by trace amphipathic contaminants.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram of a long chain organic acid with a hydrophilic headand a hydrophobic tail.

FIG. 2 is a diagram showing long chain organic acid forming asublimation impending monolayer.

FIG. 3 is a diagram of a citric acid molecule with multiple carboxylateheads and no hydrophobic tails.

DETAILED DESCRIPTION

FIG. 1 is a diagram of a long chain organic acid with a hydrophilic headand long hydrophobic tail. FIG. 2 illustrates a plurality of attachedlong chain organic acid tails that become entangled or intertwined toform a sublimation impeding monolayer 11. The sublimator porous surfaceplate 13 has the hydrophilic heads 18 attached due to the porous platecationic charge 17, thus preventing water 19 from sublimating thoughplate 13. In an example, the hydrophilic heads 18 are anioniccarboxylate “heads” of amphipathic contaminants.

In an embodiment, a two-step process to passivate a porous metal platein order to maintain a sublimation capability of the plate isillustrated. Step one of this invention involves a high temperatureoxidation of the metallic plate of the sublimator in a flowing stream ofoxygen for a minimum of four hours. The temperature ranges from about900° F. (482° C.) to about 1200° F. (649° C.). Longer times are alsocontemplated. The sublimator plate is typically made from stainlesssteel and is porous as is standard in the industry. The presentinvention is also useful for nickel based alloy porous plates. This stepconverts positively charged cations of the metal cationic surface to anuncharged oxide, exemplified by the reaction series below.

Charged nickel cation on the surface of a stainless steel porous platereacts:

-   -   Ni(s)-------→Ni²⁺+2e.

Oxygen in the presence of heat reacts:

-   -   O(g)+2e⁻---→O².

Nickel passivated as nickel oxide

-   -   2Ni(s)+O_(2(g))----→2NiO(s).

The high temperature oxidation (step one) is then followed with a steptwo, where a short-chain molecule with multiple functional carboxylategroups and no hydrophobic tail is used to render the balance of metalcation groups passive. The short chain molecule may be any four carbonor less organic acids with one or more carboxylate groups.

Citric acid has been found to be effective as the short chain moleculein step two. A citric acid pickling is accomplished by submerging theporous plate in a low concentration of citric acid for a minimum of twohours or flowing through the porous plate. A typical concentration ofcitric acid in water would be 0.1 molar or less. Citric acid isenvironmentally relatively benign and remains present and bound underoperating conditions of about less than 120° F. (48.9° C.) due to itslow vapor pressure. Citric acid decomposes at 307° F. (152.8° C.).

With the present invention high temperature oxidation (step one) andshort chain molecule addition such as citric acid (step two), multiplefunctional carboxylate groups attach without becoming intertwined, asseen in FIG. 3. The sublimation impeding hydrophobic monolayer permitsthe use of other water sources for the space suit or space station.

While the invention has been described with reference to an exemplaryembodiment(s), it will be understood by those skilled in the art thatvarious changes may be made and equivalents may be substituted forelements thereof without departing from the scope of the invention. Inaddition, many modifications may be made to adapt a particular situationor material to the teachings of the invention without departing from theessential scope thereof. Therefore, it is intended that the inventionnot be limited to the particular embodiment(s) disclosed, but that theinvention will include all embodiments falling within the scope of theappended claims.

The invention claimed is:
 1. A sublimator porous plate comprising: aporous metal plate having a porous passivated surface consistingessentially of: a first passivated portion including an uncharged oxide;and a second passivated portion including a plurality of short chainmolecules attached to metal cations, each short chain molecule havingmultiple functional carboxylate groups and no hydrophobic tail.
 2. Thesublimator porous plate of claim 1, wherein the uncharged oxide isformed at an elevated temperature, and the elevated temperature rangesfrom about 900° F. (482° C.) to about 1200° F. (649° C.), wherein theplurality of short chain molecules include a four carbon or less organicacid having at least one carboxylate group.
 3. The sublimator porousplate of claim 1, wherein the porous plate is made from a metal selectedfrom the group consisting of stainless steel and nickel based alloys. 4.The sublimator porous plate of claim 3, wherein the porous plate is madefrom stainless steel.
 5. The sublimator porous plate of claim 2, whereinthe porous plate is heated to maintain the elevated temperature for atleast four hours.
 6. The sublimator porous plate of claim 1, wherein theplurality of short chain molecules include citric acid.
 7. Thesublimator porous plate of claim 1, wherein the porous plate issubmerged in a dilute solution of citric acid for at least two hours. 8.The sublimator porous plate of claim 7, wherein the dilute solution isless than 0.1 molar citric acid in water.